1. Field of the Invention
The present invention relates to processes for treating paper pulp and particularly relates to a method for enzyme treatment of paper pulp. One of the biggest challenges facing the pulp and paper industry is to reduce the use of chlorine in the bleaching process. The effluent from the pulp bleaching plant, that portion of a mill that converts brown pulp to white, contains numerous chlorinated organic substances including toxic chlorinated phenols and dioxin. Pulp and paper processors worldwide are under intense regulatory pressure to reduce these emissions.
The present invention relates to an improved method for using enzymes in the processing of paper pulp to boost the efficiency of the bleaching process. The process of this invention overcomes a critical and heretofore unrecognized problem that has reduced the effectiveness of enzymes in a conventional pulp mill bleachery. The invention makes possible a three to four fold improvement in the "brightness boosting" power of these enzymes as well as reduced chlorine requirements.
2. Background of the Invention
The starting point for making paper is wood. Wood consists primarily of cellulose, hemicellulose, and lignin. The manufacture of high quality, bright white paper largely depends on removing the lignin from the wood pulp with minimal degradation to the cellulose and hemicellulose. Although lignin is present in lower grades of paper such as newsprint, complete lignin removal is essential for the production of fine paper. This is because lignin weakens and imparts color onto the pulp. The most common method for producing strong pulp that is light in color for high quality paper is the Kraft process. In North America, for example, 32.8 million tons of bleached Kraft pulp are presently produced annually for paper manufacture.
In conventional Kraft pulping, 80% to 95% of the lignin is removed from the wood by cooking it in an alkali liquor. After being washed with water, the cooked material contains 1.5% to 5% residual lignin and is known as brownstock. The remaining lignin is removed by a multistage bleaching process to obtain a bright, stable final product.
The first two stages of a conventional bleaching process involve treating the brownstock with chlorine, and then extracting the pulp with sodium hydroxide. These chlorine and extraction stages reduce the lignin concentration in the pulp to less than 1% and are known as the "delignification" stages. After delignification, the final remaining lignin in the pulp is removed by treating it with oxidizing chemicals such as chlorine dioxide, sodium hypochlorite and sodium hydrosulphite. These treatment stages are known as the "brightening" stages because the final product is the desired bright white pulp.
Unfortunately, the effluent from this chlorine-based bleaching process contains several classes of toxic compounds, namely organochlorines. These compounds are formed principally when chlorine reacts with lignin in the first bleaching stage. The organochlorine production by Kraft mills has been expressed in two ways: adsorbable organic halides (AOX) and dioxin level.
AOX is a nonspecific measure of the total organochlorine production of a mill, and is generally 1.5 to 8 kg per ton (T) of pulp produced, or 1 to 10 T/day for most mills. Although the link between AOX and toxicity is not clear, there is recent evidence that the LD.sub.50 for trout is 50 ppm AOX in wastewater (Cook et al., Pulp and Paper Canada 91:8, 1990). Dioxin is a specific compound that accounts for about 1/1000 of the AOX. Dioxin is one of the most acutely toxic compounds known, and has been found in mill effluents, in the pulp itself, in finished pulp products (coffee filters, milk cartons, diapers, writing paper), and in the food chain (including trout and crab) where dioxin bioaccumulates to levels thousands of times higher than in pulp wastewater.
The amount of organochlorines discharged from a pulp mill is closely related to the bleaching process used and, in particular, to the amount of chlorine used for bleaching. The following relationship between AOX production and bleaching chemical usage has been recognized: EQU AOX=0.12(C+H/2+D/5) (1)
where the AOX discharge is expressed in kg/T pulp, C is the chlorine charge (kg/T pulp), H is the hypochlorite charge (kg active chlorine/T pulp) and D is the chlorine dioxide charge (kg active chlorine/T pulp) (Germgard et al., Paperi ja Puu, 4: 287-290, 1983).
Some of the present technologies recognized to reduce chlorine usage include:
1. Extended delignification. This method involves prolonging the kraft pulping process to enhance lignin removal before bleaching. The lignin content of softwood brownstock is thereby reduced from 4% to 3%, which in turn reduces chlorine levels and AOX discharges by 20%. Extended delignification techniques involve additional digester capacity, which is prohibitively expensive for existing mills. This option is only appropriate for new mills. PA1 2. Oxygen delignification. The use of oxygen gas to treat the pulp before the C stage can reduce the lignin content of softwood brownstock from 4% to 2%, thereby reducing AOX discharges by up to 50%. Oxygen delignification, however, is an extremely capital intensive operation, costing as much as $20 to $50 million. PA1 3. High chlorine dioxide substitution. The substitution of chlorine dioxide for chlorine in the C stage can reduce the AOX discharge by up to 50%. The capital cost of installing chlorine dioxide generators, however can be over $10 million for mills without the existing equipment. The high cost of chlorine dioxide could be expected to add $12/T or more of bleaching chemical cost at 100% substitution for chlorine.
Clearly, these alternatives incur significant costs. One of the primary objectives of this invention is to provide an improved way of using enzymes as part of the bleaching process to make it practical to reduce AOX discharges without incurring significant capital expenses.
Enzymes are biological catalysts, i.e., they are proteins with molecular weights ranging from 12,000 to 200,000 daltons that accelerate specific chemical reactions without being consumed in the overall process. They typically work in aqueous media, at atmospheric pressure, and at mild temperatures ranging 20.degree. C. to 60.degree. C.
Enzymatic catalysis involves the formation of an intermediate complex between the enzyme and its substrate. The region of an enzyme that specifically interacts with the substrate is called the active site. On binding to this site, the substrate is brought into close proximity to specific groups on the enzyme that cooperatively destabilize certain bonds in the substrate, making them more chemically reactive.
Enzymes differ most strikingly from ordinary chemical catalysts in their substrate specificity and catalytic efficiency. Most enzymes have only a few natural substrates, which are converted to single products in remarkably high yields. The unique structures of the active sites of enzymes provide this specificity and not only allow favorable binding of specific substrates but also exclude the unfavorable binding of many substances that are not substrates. There are strong attractive non-covalent forces between the active site and a substrate, and enzymes may be thought to act by "attracting" the substrate into the site, where the extraordinarily unique structural transformations of the substrate occur. For enzyme systems, a high degree of specificity is maintained, with the reaction proceeding 10.sup.6 to 10.sup.12 times faster than the spontaneous, uncatalyzed reaction in aqueous solution.
The pH has a marked influence on the rate of enzymatic reactions. Characteristically, for each enzyme there is a pH value at which the rate of reaction is optimal, and on each side of this optimum, the rate is lower. The influence of pH on enzymatic reactions may involve several different types of effects. Enzymes, like other proteins, are ampholytes and possess many ionic groups. If enzymatic function depends on certain special groupings, these may have to be present in some instances in the un-ionized state and, in others, as ions. In some cases, the groups in the active site of the enzyme that are responsible for catalytic action have even been identified by comparing the effect of pH on enzymatic activity and the known pK values of titratable groups in the protein. The pH may also influence the rate of enzymatic reaction indirectly insofar as many enzymes, like proteins in general, are stable only within a relatively limited pH range.
The use of enzymes to reduce chlorine requirements in pulp bleaching has been known and involves the treatment of brownstock with a class of enzymes, known as hemicellulases, that hydrolyze the hemicellulose portion of wood pulp. Hemicellulose in wood pulp consists of two types of structures with polysaccharide backbones: xylan and glucomannan. Xylan, which forms 90% of the hemicellulose in hardwood and 50% of that in softwood, is substituted with arabinosyl, acetyl, and other side groups. Glucomannan is found primarily in softwood. The enzymes that have shown benefit in bleaching include xylanase, arabinase and mannanase (Paice, et al., Biotechnology and Bioengineering, 32:235-239, 1988; Viikari, et al., Biotechnology in the Pulp and Paper Industry, The 3rd International Conference, Stockholm Jun. 16-19, 1986; Preliminary Product Information, Pulpzym.TM. Novo Enzyme Process Division, 1989; Kantelinen et al. International Pulp Bleaching Conference, Jun. 5-9 1988, TAPPI Proceedings pp. 1-9); i.e., enzymes that hydrolyze xylan, araban, and mannan linkages. Each of these enzymes catalyze a specific and known chemical reaction, hydrolysis. It is therefore generally believed that enzymes enhance the extractability of lignin by partially hydrolyzing the hemicellulose portion of unbleached pulp. This, in turn, leads to a significantly reduced chlorine requirement to bleach pulp.
In this regard, studies have reported linkages between hemicellulose, particularly xylan, and lignin (in wood) (Eriksson, et al., Wood Sci. Technol. 14:267-279 1980). The two types of linkages that have been shown are ester linkages between ligninand the methylglucuronic acid residues of xylan (Das, et al., Carboh. Res. 129: 197-207, 1984), and ether bonds from lignin to hydroxyl moieties of the arabinosyl side groups of xylan (Joseleau et al., Svensk Papperstidn, 84: R123, 1981). It has been hypothesized that by hydrolyzing hemicellulose, these enzymes act to "release" lignin from chemical linkages to the fiber being bleached.
A number of microorganisms are known to make hemicellulase enzymes. Xylanolytic enzymes (xylan attacking enzymes including xylanase and arabinase) are produced by microorganisms including Trichoderma reesei, Aspergillus awamori, Streptomyces olivochromogenes, and Fusarium oxysporum (Poutanen, et al., Appl. Microbiol. Biotechnol. 23:487-490, 1986; Poutanen, et al., J. of Biotechnology, 6:49-60, 1987; European Patent Application 0373107, filed published Jun. 13, 1990). Mannanase enzymes are made by Trichoderma and Aspergillus sp., among others (Kantelinen, Kemia-Keemi 3: 228-231), 1988). This invention is particularly concerned with the use of so-called "acid" hemicellulase enzymes, i.e., enzymes whose optimum activity is at pH levels ranging from 3 to 6.
The use of hemicellulases to enhance the bleaching of pulp has been reported by researchers at VTT in Finland, the Pulp and Paper Research Institute in Canada, and Novo in Denmark. In these studies, unbleached pulp was treated with enzymes before the addition of the bleaching chemicals. Enhanced bleaching by enzymes is quantified by the increased brightness of enzyme-treated pulp (after bleaching) relative to pulp bleached without enzyme treatment. Brightness is measured by a standard brightness meter and expressed on the ISO scale. A highly reflective barium sulfate surface for example, is 99 ISO brightness, fine writing paper about 90 ISO brightness, and newspaper 65 ISO brightness.
VTT reported that treatment of pulp with hemicellulases from Aspergillus awamori and Streptomyces olivochromogenes increased the brightness of the pulp after bleaching by up to 5 ISO points (Viikari, et al., Biotechnology in the Pulp and Paper Industry, The 3rd International Conference, Stockholm June 16-19, 1986; Viikari, et al., 1987; Kantelinen, International Pulp Bleaching Conference, Jun. 5-9 1988, TAPPI Proceedings pp. 1-9). This corresponded to a 25% decrease in the amount of chlorine required to reach a given ISO brightness. Both of these hemicellulases were classified as xylanases, because xylanase was putatively the active enzyme that enhanced bleaching. VTT also showed enhanced bleaching with xylanase from Aspergillus niger and Trichoderma reesei and from Bacillus subtilis and arabinase from Trichoderma reesei (Kantelinen, International Pulp Bleaching Conference, Jun. 5-9 1988, TAPPI Proceedings pp. 1-9).
Paice, et al., Biotechnology and Bioengineering, 32:235-239, 1988, at Paprican showed that treating unbleached pulp with xylanase enzyme from Schizophyllium commune increased the brightness of the pulp (after bleaching) by 7 ISO points.
All of these studies carried out the enzyme treatment of pulp at pH 5, which is recognized as the optimum for the activity of these enzymes. The optimum pH for the xylanase enzymes is determined by isolating the substrate for the enzyme, in this case xylan, and measuring the ability of the enzyme to hydrolyze it in a dilute buffer solution. The term "pH optimum" is used to mean the pH at which a hemicellulase enzyme has optimum activity for the hydrolysis of its natural hemicellulase substrate in a dilute buffer solution. For example, the optimum pH for T. reesei xylanase is 4 to 5 and is measured by its ability to hydrolyze xylan (Dekker, Biotechnology and Bioengineering, Vol. XXV:1127-1146, 1983; Poutanen, et al., J. of Biotechnology 6:49-60, 1987; Preliminary Product Information, Pulpzyme.TM., Novo Enzyme Process Division, 1989). For A. awamori xylanase, the pH optimum is 5.0 (Poutanen, et al., J. of Biotechnology 6:49-60, 1987), for A. niger xylanase, the pH optimum is 4 to 5 (Conrad, Biotechnol. Lett. 3:345-350, 1981), and for S. olivochromogenes xylanase, the pH optimum is 6.0 to 6.5 (Poutanen, et al., J. of Biotechnology 6:49-60, 1987). The procedure of Ebringerova, et al., Holzforschung 21:74-77, 1967, has, for example, been used to isolate xylan from birch, beech, larchwood, and other sources while minimizing changes to the xylan structure. The isolated xylan is therefore of similar structure to the indigenous xylan in wood pulp. All of the enzyme treatments by VTT and Paice, et al. were carried out at pH 5 to be in the range of optimum activity for the xylanase enzymes.
Novo-Nordisk has described the effect of pH on the activity of its enzyme preparation, Pulpzyme.TM. HA. Pulpzyme.TM. HA is a xylanase preparation derived from a selected strain of Trichoderma reesei in which the enzyme preparation has endo-1,4-beta-D-xylanase, and exo-1,4-beta-D-xylanase activities, and a certain amount of cellulase activity. Pulpzyme.TM. HA is described by Novo as having been standardized to 500 XYU/g, with one xylanase unit (XYU) defined as the amount of enzyme, under standard conditions of pH 3.8, 30.degree. C., 20 minute incubation, that degrades larchwood xylan to reduce carbohydrates with a reducing power corresponding to 1 .mu.mol xylose. Pulpzyme.TM. HA further contains approximately 300 EGU/g, in which one endo-glucanase unit (EGU) is the amount of enzyme, under standard conditions of pH 6.0, 40.degree. C., 30 minute incubation, that lowers the viscosity of a carboxymethyl cellulose solution to the same level as an enzyme standard defining 1 EGU. For the NOVO Pulpzyme.TM. HA, the optimum pH for its performance is pH 4 to 5, and the activity at pH 7 is only 40% of the optimum. Because Kraft brownstock usually has a pH in excess of 9, Novo suggests that the pH of the pulp be adjusted to 5 to 6 for xylanase treatment.
Pulpzyme.TM. HA contains significant amounts of cellulose degrading activity, in addition to its xylanase activity. This cellulase enzyme can have very undesirable effects on pulp qualities such as pulp strength. As FIG. 1 shows, however, this problem with Pulpzyme.TM. HA can be slightly ameliorated by recognizing that the potency of xylanase increases relative to cellulase as the pH is increased from 5.5 up to 6.5. By selecting process conditions such as pH 6.5, therefore, Novo suggests that the undesirable effects of cellulase can be reduced. Operating at an elevated pH, however, is done at the expense of a significant reduction in the brightness boosting of the xylanase. Novo teaches that this compromise pH 6.5 level must not be exceeded because "the enzyme is rapidly inactivated above pH 7-8". (Preliminary Product Information, Pulpzyme.TM. Novo Enzyme Process Division, 1989, at page 3).
In the present invention, a high level of brightness boosting activity is achieved at pH levels previously taught by Novo to inactivate the enzymes. Moreover, in one preferred embodiment, this invention comprises the use of enzyme preparations with low contaminating cellulase levels, i.e., much lower than Pulpzyme.TM. HA. Accordingly, the Novo teachings of ways to deal with contaminating cellulase are therefore irrelevant to this embodiment.
The pH optima for enhancing bleaching with xylanase of around 5.0 taught by Novo and other workers has been confirmed by our own testing using Kraft brownstock that has been well washed with water. FIG. 2 (from our Example 4) compares the activity profile taught by Novo with the brightness boosting performance of a Trichoderma xylanase. As one would expect, the performance of xylanase to brighten pulp drops off significantly as the pH of the pulp is increased, to the point where less than 40% of the maximum brightness boosting is achieved at pH levels over 7.0.
The prior teachings for using enzyme preparations that are substantially free of contaminating cellulase activity in bleaching are absolutely clear on one significant point. They teach that the operating pH should be in the range of 5 to 6 and preferably as close as possible to that of the enzyme's pH optima for hydrolysis.
While the laboratory testing by Novo Nordisk and that shown in FIG. 2 has been conducted with well washed brownstock, most brownstock in commercial mills is not well washed. Operating pulp mills must make compromises between the costs and benefits of washing. As a result, one would typically expect to find significant levels of residual Kraft black liquor in the pulp being sent to the pulp bleachery of an operating mill. The degree of washing is usually assessed by measuring the residual soda in the pulp. While the well washed samples of brownstock used in our laboratory testing had residual soda levels below 1 Kg per ton, one often finds residual soda levels ten times this high in operating mills.
Not surprisingly, residual black liquor is deleterious to the action of xylanase enzymes. The inventors have found, for example, that the conventional treatment conditions used to obtain a peak brightness boost of 7.5 ISO points achieves only a 1 to 2 ISO point brightness boost when applied to brownstock taken directly from the last washing stage of an operating kraft mill, i.e., imperfectly washed material.